Increasingly stringent environmental regulations will require significant reductions in the sulfur content of transportation fuels. For example, by the end of this decade, maximum sulfur levels for distillate fuel will be limited to 10 wppm in Europe and Japan and 15 wppm in North America. To meet these ultra-low sulfur requirements without expensive modifications to existing refineries, it will be necessary to design a new generation of catalyst that has very high activity for desulfurization, particularly for distillate fuels at low to medium pressure. The issue of sulfur contamination is further complicated by the need of many refineries to process crudes that are high in both sulfur and nitrogen contaminants as the supply of crudes low in these contaminants is decreasing.
The ease of removal of sulfur contaminants depends in part on their chemical nature. For example, sulfides and disulfides present in lower boiling distillate fractions are generally more easily removed than the thiophenic and other heterocyclic sulfur compounds present in higher boiling fractions. Thiophenic and other heterocyclic sulfur compounds require both ring breaking and saturation and therefore may require more active hydrodesulfurization (HDS) catalysts and/or more severe reaction conditions.
The general approach to both the HDS and hydrodenitrogenation (HDN) of distillate fuels is catalytic hydrotreating to convert sulfur and nitrogen contaminants to hydrogen sulfide and ammonia. Typical commercial hydrotreating catalysts include Group VIB and Group VIII metals. Molybdenum and tungsten are the most commonly used Group VIB metals while cobalt and nickel are the most commonly used Group VIII metals. These catalysts are usually used in their sulfided form, and normally comprise a carrier having deposited thereon the metal components. CoMo catalysts on alumina are recommended for low pressure processes whereas NiMo catalysts on alumina are recommended for high pressure processes.
Various approaches have been tried for providing HDS catalysts with improved activity. In one approach, a family of compounds, related to hydrotalcites, e.g., ammonium nickel molybdates, has been prepared. Whereas X-ray diffraction analysis has shown that hydrotalcites are layered phases composed of positively charged sheets and exchangeable anions located in the galleries between the sheets, the related ammonium nickel molybdate phase has molybdate anions in interlayer galleries bonded to nickel oxyhydroxide sheets. See, for example, Levin, D., Soled, S. L., and Ying, J. Y., Crystal Structure of an Ammonium Nickel Molybdate prepared by Chemical Precipitation, Inorganic Chemistry, Vol. 35, No. 14, p. 4191-4197 (1996). The preparation of such materials also has been reported by Teichner and Astier, Appl. Catal. 72, 321-29 (1991); Ann. Chim. Fr. 12, 337-43 (1987), and C. R. Acad. Sci. 304 (II), #11, 563-6 (1987) and Mazzocchia, Solid State Ionics, 63-65 (1993) 731-35.
Another approach is disclosed in U.S. Pat. Nos. 6,162,350; 6,652,738, 6,635,599 and 6,534,437, all of which are incorporated herein by reference, which relates to a family of bulk Group VII/Group VIB trimetallic catalysts for the removal of sulfur from distillate fuels. The preferred trimetallic catalysts are comprised of Ni—Mo—W and are prepared from a variety of catalyst precursor compounds.
Yet another approach is to combine the hydrotreating catalyst with additives. Examples of this approach are found in U.S. Pat. Nos. 6,923,904 and 6,280,610. U.S. Pat. No. 6,280,610 discloses a processes for reducing the sulfur content of a hydrocarbon feedstock which comprises subjecting a catalyst comprising a Group VIB metal component, a Group VIII metal component, and an organic additive on a carrier to an optional sulfidation step, and contacting a feedstock with the sulfided catalyst. In U.S. Pat. No. 6,923,904, the sulfidation step is not optional. In both patents, the catalyst is formed by impregnating a support with a Group VIB metal component, a Group VIII metal component, and an organic additive. The impregnated support is then heated to a temperature sufficient to maintain at least a portion of the additive on the support by avoiding decomposition or evaporation.
There is still a need in the art for even more active catalysts for producing transportation fuels having ultra-low levels of sulfur, particularly for low to medium pressure hydrotreating.